Tire tread rubber composition comprising rice husk ash silica
This invention is based upon the unexpected discovery that rice husk ash silica (synthetic amorphous silica from rice husk ash) provides an exceptionally good combination of properties when used in certain tire tread rubber formulations, such as an excellent combination of traction, tread-wear, and rolling resistance. These tread rubber formulations are comprised of a blend of high cis-1,4-polybutadiene rubber and a solution styrene-butadiene rubber which is preferably functionalized with amine groups, siloxy groups, thiol groups, carboxyl groups, or groups that are reactive with hydroxyl groups present on the precipitated rice husk ash silica. The use of such rice husk silica as a reinforcing-filler in rubber formulations is also highly desirable because such silica is a sustainable resource (bio-renewable) which is derived from an agricultural waste material which would otherwise be discarded as an undesirable waste product.
1 . A sulfur curable rubber formulation which is comprised of (1) a styrene-butadiene rubber, (2) a high cis-1,4-polybutadiene rubber, (3) rice husk ash silica, wherein the rice husk ash silica has a BET nitrogen surface area which is within the range of 120 m 2 /gram to 300 m 2 /gram, a CTAB surface area which is within the range of 100 m 2 /gram to 300 m 2 /gram, a ratio of BET to CTAB surface areas which is with the range of 0.8 to 1.3, a bulk density which is within the range of 120 grams/liter to 200 grams/liter, a loss on ignition which is within the range of 3.0% to 6.0%, and a heat loss which is within the range of 4.0% to 7.0%, (4) 2 phf to 15 phf of a silica coupling agent which comprises a hydrophobating silane and a sulfur crosslinkable silane and wherein the sulfur crosslinkable silane is present in the amount from 0.5 to 5 phf and the weight ratio of the hydrophobating silane to the sulfur crosslinkable silane ranges from 4:1 to 20:1; wherein the sulfur curable rubber formulation is void of titanium compounds.
2 . The sulfur curable rubber formulation of claim 1 wherein said sulfur curable rubber formulation is further comprised of sulfur.
3 . The sulfur curable rubber formulation of claim 1 wherein the styrene-butadiene rubber is functionalized with carboxyl groups.
4 . The sulfur curable rubber formulation of claim 1 wherein the sulfur crosslinkable silane is present at a level which is within the range of 1 phf to 3 phf; wherein total level of the hydrophobating silane and the sulfur crosslinkable silanes is within the range of 6 phf to 15 phf, and wherein the rice husk ash silica is present at a level of about 65 parts by weight per 100 parts by weight of rubber.
5 . The sulfur curable rubber formulation of claim 1 wherein the hydrophobating silane consists of organosilicon compounds.
6 . The sulfur curable rubber formulation of claim 1 wherein said sulfur curable rubber formulation is void of natural rubber and synthetic polyisoprene rubber.
7 . The sulfur curable rubber formulation of claim 1 wherein the weight ratio of the hydrophobating silane to the sulfur crosslinkable silane is within the range of 5:1 to 12:1.
8 . The sulfur curable rubber formulation of claim 1 wherein the weight ratio of the hydrophobating silane to the crosslinkable silane is within the range of 7:1 to 9:1.
9 . The sulfur curable rubber formulation of claim 1 wherein the sulfur curable rubber formulation is void of rubbery polymers other than the styrene-butadiene rubber and the a high cis-1,4-polybutadiene rubber, wherein the styrene-butadiene rubber is present at a level which is within the range of 55 weight percent to 90 weight percent, and wherein the a high cis-1,4-polybutadiene rubber is present at a level which is within the range of 10 weight percent to 45 weight percent, based upon the total weight of rubber in the formulation, and wherein the styrene-butadiene rubber is functionalized with carboxyl groups.
10 . The sulfur curable rubber formulation of claim 1 wherein the sulfur crosslinkable silane is of the structural formula:
HS—(CH 2 ) n —Si—[(O—(CH 2 ) m CH 3 )] 3
wherein n represents an integer from 1 to 6 and wherein m represents an integer from 0 to 6.
11 . The sulfur curable rubber formulation of claim 10 wherein n represents the integer 3 and wherein m represents the integer 1.
12 . The sulfur curable rubber formulation of claim 1 wherein the hydrophobating silane is of the structural formula:
CH 3 —(CH 2 ) n —Si—[(O—(CH 2 ) m CH 3 )] 3
wherein n represents an integer from 0 to 18 and wherein m represents an integer from 0 to 6.
13 . The sulfur curable rubber formulation of claim 12 wherein n represents the integer 7 and wherein m represents the integer 1.
14 . The sulfur curable rubber formulation of claim 1 wherein the hydrophobating silane consists of organozirconium compounds.
15 . The sulfur curable rubber formulation of claim 14 wherein the organozirconium compound is selected from the group consisting of tetra (C1-C18) alkoxy zirconates, phenyl zirconium (iv) trichloride, methyl zirconium (iv) trichloride, ethyl zirconium (iv) trichloride, propyl zirconium (iv) trichloride, methyl zirconium (iv) tribromide, ethyl zirconium (iv) tribromide, propyl zirconium (iv) tribromide, and chlorotripentyl zirconium (iv).